The larger size could be easily pinned to show you how it would work. Altering the shoulder, bust. the arms and the sleeve will be necessary, but that is easy to do by an experienced tailor. When I have sized up for jacket length, I have to do all of the above and sometimes it takes 2-3 fitting to get right, but it is worth it.Now we’re talking - you’re giving me hope. I’ll go and check out this option. At least you’ve set a precedent for me!
Edited to add: Please see previous post about nipping it in.